This invention relates to methods of and apparatus for the pumping of transition metal ion containing solid state lasers using visible diode laser sources operating in the visible spectrum.
Solid state lasers employ dopant ions incorporated in dilute concentrations in solid hosts as the laser-active gain media. Broadly tunable solid state lasers derive their tunability from emission of vibrational quanta (phonons) concurrent with the emissions of light quanta (photons). The energies of the photons and phonons which are emitted simultaneously in a vibronic laser add up to the energy of the associated purely electronic or "no-phonon" transition. The broad wavelength tunability of such a "vibronic laser" derives from the broad energy phonon continuum which complements the photon emission.
The use of a transition metal ion as a dopant in a solid state laser medium is known in the art. Walling et al (U.S. Pat. No. 4,272,733) discloses the use of alexandrite, a chromium-doped beryllium aluminate (Cr.sup.+3 : BeAl.sub.2 O.sub.4) crystal, as a laser medium. The disclosure of Walling et al U.S. Pat. No. 4,272,733 is incorporated herein by reference.
Alexandrite has been optically pumped with flashlamps in pulsed operation (See Tunable Alexandrite Lasers, IEEE, J. of Quantum Electronics, Vol. QE-16, No. 12, Dec. 1980, pp. 1302) and arc-lamps in continuous wave ("CW") operation. (See Tunable CW Alexandrite Laser, IEEE, J. of Quantum Electronics, Vol. QE-16, No. 2, Feb. 1980, pp. 120). Such flashlamps and arc-lamps have broad band emissions ranging from ultraviolet (300 nm) to infared (1000 nm). Alexandrite, however, predominantly absorbs in the visible wavelength region, approximately 400-700 nm. The overlapping of alexandrite's absorption spectrum by the output of such lamps results in good but, not ideal, pumping efficiencies and in substantial heating and consequential thermo optical effects.
The pumping of Neodymium ion (Nd.sup.+3) solid state lasers by semiconductor laser diodes had been demonstrated in the 1970's (see W. Koechner, "Solid State Laser Engineering", Springer series in Optical Sciences, vol. 1, chapt. 6, Springer-Verlag, NY, 1976). However, it was not thought to be a practical means for excitation because of limitations on the laser diodes. In the mid 1980's the advances in semiconductor diodes lasers improved their power and reliability, and several to many milliwatt diode lasers became routinely available. Initially these higher power diode lasers were Gallium Arsenide GaAs compositions emitting at wavelengths longer than 1 um. Their development was driven by the communications industry interested in 1.3-1.5 um fiber optic communications networks. As the semiconductor growth techniques, primarily molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), matured it became possible to grow and fabricate other III-IV composition laser diodes, notably the ternary compositions of AlGaAs, lasing at wavelengths as short as 700 nm, but with power and lifetime optimized at wavelengths longer than 750 nm. In 1984 and 1985 diode lasers were used to pump Nd:YAG (ytterium aluminum garnet) lasers in laboratory demonstrations of practical devices (see R. Scheps, "Efficient laser diode pumped Nd lasers", Applied Optics, Vol. 28, No. 1, 1 Jan. 1989, pp. 8-9). Pumping wavelengths were near 820 nm where the Nd.sup.+3 absorption bands were well matched to the highest power output wavelengths of the AlGaAs diodes.
Prior to this invention, it was believed that diode pumping of transition metal ion-containing laser materials in general and tunable vibronic laser materials in particular would be impractical for two reasons: 1) the absorption bands are much broader in the transition metal ion-containing laser materials than in the rare earth ions like Nd.sup.+3 and the absorption strength is weaker unless pumping is done at short (visible or near visible) wavelengths (typically 700 nm and shorter) and 2) the emission cross section of tunable vibronic laser media is typically substantially lower than Nd.sup.+3 media because the oscillator strength is by necessity spread out over the tuning range rather than localized to a specific wavelength or few wavelengths. This means that the laser gain is substantially lower for a given excitation level and the laser threshold therefore substantially higher for a given loss.